Recently, technique adapted to detect the terrestrial magnetism has come to be required to highly sensitively and accurately detect a very weak magnetic field so as to broaden the scope of application. Magnetic impedance (MI) elements have been attracting attention as magnetic field detecting elements of this type. With a known magnetic field detecting method using MI element, the magnetic field is detected by directly applying a high frequency electric current to a magnetic element and detecting the voltage signal generated by the detection coil wound around or arranged in the vicinity of the magnetic element.
FIG. 1 illustrates a basic circuit diagram of a circuit that can be used for detecting magnetic field. Referring to FIG. 1, a pulse oscillation is generated by means of an oscillation circuit 11 surrounded by broken lines and an electric current is made to flow to MI element 14 by way of inverter 12 and current regulating resistor 13. Then, the change in the magnetic flux caused by the MI element 14 is taken out as a change in the voltage generated in detection coil 15 by winding the detection coil 15 around the MI device 14. One of the opposite ends of the detection coil 15 is grounded while the other end is connected to waveform detection circuit 16 formed by a peak detection diode and a RC circuit so that an amplitude-modulated magnetic field signal is taken out from the waveform detection circuit 16. Alternatively, the magnetic field signal may be detected by synchronous detection substantially in synchronism with rises and falls of oscillation of the oscillation circuit 11 by means of an analog switch with a hold capacitor. Then a voltage Vso for a zero external magnetic field is characterized and a reference voltage that matches the voltage Vso is selected by means of an amplifier 17 having a variable resistor 18 inserted between the power supply voltage and the grounding terminal. Thus the output voltage is regulated manually at the output of the amplifier.
However, the characterized Vso usually changes as the ambient environment changes. Under such circumstances, it is difficult to manually regulate the output voltage. The sampling jitter for peak detection causes much higher signal variation if the signal on the detection coil is sharp at the peak top. It is also unable to optimize signal detection if magnetic field is not smooth or magnetic field changes dramatically, even coupled with nonlinear effect. Therefore the magnetic detection circuit for detecting magnetic field is unable to detect very weak magnetic field especially for sub mG (milliGauss) magnetic field or detect noisy magnetic field. As a result this detection technique will cause large error in modern application especially for application such as air mouse, gyro, etc.
The existing technologies of detecting magnetic field have drawbacks of being unable to handle conditions such as element intrinsic noise of an oscillation circuit, sampling jitter induced noise of peak voltage variation, coil loading effect that affects non-external magnetic field characterization, weak magnetic field, etc. Thus, it is imperative to devise a technology of detecting magnetic field having high flexibility and reliability.